In a well known process, a thin aluminum layer is deposited by sputtering onto a substrate. Atoms of aluminum from a target are ejected from the target by impact from an ionized gas. The aluminum atoms are then deposited on the surface of the substrate.
When a thin metal layer, particularly soft metal such as aluminum with a high coefficient of thermal expansion is sputtered onto a substrate with a low coefficient of thermal expansion, such as silicon or silicon dioxide, microscopic protrusions often appear in the surface of the metal layer. Such protrusions commonly appear in the aluminum metallization layers deposited on oxidized silicon surfaces in the manufacture of integrated circuits. Protrusions appearing immediately after deposition of the aluminum layer are termed "growth hillocks". The protrusions which develop after cycling the integrated circuit to a high temperature during manufacturing steps are called "annealing hillocks".
In either case, these microscopic protrusions or hillocks are troublesome and can cause subsequent device failure. For example, hillocks can cause shorts between conductive layers in a device in areas where conductors cross over one another, or in elements of the device having two layers of conductors such as integrated capacitors. More particularly, if an insulating layer, e.g. SiO.sub.2 is formed on an aluminum layer at a thickness of less than 1.mu. metal hillocks greater than 1.mu. will protrude through the SiO.sub.2 layer and contact any subsequently deposited metal layers, causing a short circuit. The protrusion of the hillocks potentially causes short circuits between conductive layers in a device at positions where the conductors cross over each other, or in devices where there are two spaced apart layers such as in capacitors, forming part of an integrated circuit. Hillocks are also found in aluminum layers formed in optical disks.
Previous attempts to overcome the problem of hillock formation in thin aluminum layers have included the addition of impurities such as silicon, copper, silver and gold to the aluminum to immobilize the grain boundaries in the metal layer (see e.g. U.S. Pat. No. 4,012,756 issued Mar. 15, 1977 to Chaudhari et al). One suggested approach to reducing hillocks specifically in aluminum layers is to treat the layer to form a boehmite (AlO-OH) layer on its surface; see e.g. U.S. Pat. No. 3,986,897 issued Oct. 19, 1976 to McMillan et al.
Yet another disclosed approach to reducing hillocks in aluminum layers is to alternate layers of aluminum and an aluminum oxygen alloy, by periodically introducing controlled bursts of oxygen into an aluminum deposition chamber. None of these approaches has proved entirely satisfactory however.
Draper et al in an article entitled "A Hillock-Free Conductor System for Use in Multilevel Interconnects," 2nd Int'll IEEE VLSI Multilevel Interconnection Conf., Santa Clara, Calif., June 1985, pp. 90-101, discusses the use of tantalum silicide on aluminum to reduce hillocks. In multilevel devices SiO.sub.2 is frequently deposited on the metal layer and then in particular locations etched down to the surface of the metal by HF acid. Unfortunately, HF also attacks tantalum silicide and at an etch rate of 300-600 .ANG./min.
The object of this invention is to provide a simple, effective means of reducing hillock formation in aluminum layers sputtered onto a substrate eliminating the problems discussed above.